Holey electrode grids for photovoltaic cells with subwavelength and superwavelength feature sizes

ABSTRACT

A photovoltaic cell and a method of forming an electrode grid on a photovoltaic semiconductor substrate of a photovoltaic cell are disclosed. In one embodiment, the photovoltaic cell comprises a photovoltaic semiconductor substrate; a back electrode electrically connected to a back surface of the substrate; and a front electrode electrically connected to a front surface of the substrate. The substrate, back electrode, and front electrode form an electric circuit for generating an electric current when said substrate absorbs light. The front electrode is comprised of a metal grid defining a multitude of holes. These holes may be periodic, aperiodic, or partially periodic. The front electrode may be formed by depositing nanospheres on the substrate; forming a metallic layer on the substrate, around the nanospheres; and removing the nanospheres, leaving an electrode grid defining a multitude of holes on the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to solar or light energy, and morespecifically, the invention relates to photovoltaic cells.

2. Background Art

Solar energy has long been looked to as a significant part of a solutionto the ever increasing energy needs of the planet's population.Increasing costs of mining fossil fuels and increased concerns over“greenhouse” emissions have furthered interest in exploiting alternativeenergy strategies, including solar energy sources. To date, solar energyconversion has generally relied upon either the direct harvesting ofsolar thermal energy, e.g., in heating applications, or inthermoelectric conversion, or through the direct conversion of photonicenergy to electrical energy through the use of photovoltaic cells.

Photovoltaic cells have developed according to two distinct methods. Theinitial operational cells employed a matrix of single crystal siliconappropriately doped to produce a planar p-n junction. An intrinsicelectric field established at the p-n junction produces a voltage bydirecting solar photon produced holes and free electrons in oppositedirections. Despite good conversion efficiencies and long-termreliability, widespread energy collection using single-crystal siliconcells is thwarted by the exceptionally high cost of single crystalsilicon material and interconnection processing.

A second approach to produce photovoltaic cells is by depositing thinphotovoltaic semiconductor films on a supporting substrate. Materialrequirements are minimized and technologies can be proposed for massproduction. The thin film structures can be designed according to dopedhomojunction technology such as that involving silicon films, or canemploy heterojunction approaches such as those using CdTe orchalcopyrite materials. Despite significant improvements in individualcell conversion efficiencies for both single crystal and thin filmapproaches, photovoltaic energy collection has been generally restrictedto applications having low power requirements.

In all solar cells, especially the high performance ones, seriesresistance issue is a serious limiting factor for cell performance. Thisseries resistance is associated with the contact electrode geometry,emitter (top) layer sheet resistance, and metal-semiconductor contactresistance. The design of the electrode structure must consider thetrade off between area coverage and the shadowing loss. A standardapproach is to use comb-like metal grids or to use transparentconducting oxide (TCO) film that combines transparency and goodconductivity of the film. However such TCO film still has a limitedconductivity and still contributes to considerable amount of seriesresistance.

BRIEF SUMMARY

Embodiments of the invention provide a photovoltaic cell and a method offorming an electrode grid on a photovoltaic semiconductor substrate of aphotovoltaic cell. In one embodiment, the photovoltaic cell comprises aphotovoltaic semiconductor substrate having a front, light receivingsurface, and a back surface; a back electrode electrically connected tosaid back surface; and a front electrode electrically connected to thefront surface. The substrate, back electrode, and front electrode forman electric circuit for generating an electric current when saidsubstrate absorbs light. The front electrode is comprised of a metalgrid defining a multitude of holes having widths less than 1000 nms.

In one embodiment, the holes of the front electrode form a periodicpattern in the front electrode, and for example, this pattern may repeatwith a period between 200 nms and 50 ums. In another embodiment, theholes of the front electrode form an aperiodic pattern in said frontelectrode. In another embodiment, the holes in the front electrode forma partially periodic pattern in said front electrode.

In one embodiment, the holes in the front electrode have circular shapeswith diameters between 100 and 1000 nms. In one embodiment, thephotovoltaic cell is for use with light at a wavelength λ, and saidmultitude of holes in the front electrode have widths greater than λ. Inanother embodiment, the photovoltaic cell is for use with light at awavelength λ, and the multitude of holes in the front electrode havewidths less than λ. In another embodiment, the photovoltaic cell is foruse with light at a wavelength λ, and some of the holes in the frontelectrode have widths greater than λ, while others of these holes havewidths less than λ. In one embodiment, the front electrode extends overa defined area of the front surface of said substrate, and the holes ofthe front electrode cover more than 80% of said defined area.

In one embodiment of the invention, a method is provided for forming anelectrode grid on a photovoltaic semiconductor substrate. In oneembodiment, the method comprises depositing a quantity of nanoshperes onsaid substrate; forming a metallic layer on the substrate, around saidnanospheres; and removing said nanoshperes, whereby an electrode griddefining a multitude of holes is left on the substrate, electricallyconnected thereto.

In one embodiment, the depositing includes forming a monolayer of thenanospheres on said substrate. In an embodiment, the depositing includestrimming said nanospheres to a desired size; and, for example, thistrimming may be done using O₂. In one embodiment, said forming includesdepositing a layer of a metal on the substrate and the nanospheres. Inan embodiment, said removing includes lifting the nanospheres off saidsubstrate; and, for example, this may be done by peeling the nanoshperesoff said substrate using a tape and cleaning the substrate in a hotsolvent such as acetone or NMP (N-Methyl-2-pyrrolidone)

Embodiments of the invention use metal electrode to retain its very highconductivity (˜100× larger than best TCO), patterned into holeystructures with subwavelength or superwavelength feature size whilemaintaining the same level of shadowing loss. Depending on the size ofthe holes and their periodicity, the invention can achieve significantbenefits such as: reduced cell series resistance, as the metal grid haslarger area coverage so the current transport are not limited by the(high) emitter sheet resistance anymore, and enhanced light transmissiondue to subwavelength electrode linewidth (that is invisible to light) ordue to plasmonics effect.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram of a photovoltaic device.

FIG. 2( a) illustrates the impact of varying electrode thickness to theefficiency of a photovoltaic cell.

FIG. 2( b) illustrates the components of the overall series resistanceof a photovoltaic cell.

FIG. 3 illustrated the principle of replacing the fingers of aphotovoltaic cell with a holey electrode grid.

FIG. 4( a) shows the enhanced transmission effect through an array ofholes of subwavelength size.

FIG. 4( b) shows experimental data for various hole sizes.

FIG. 5 illustrates a fabrication process of a holey electrode grid.

FIG. 6 shows fabrication steps that may be used in the fabricationprocess.

FIG. 7( a) shows an etch timing diagram for trimming the sizes ofnanospheres in the fabrication process.

FIG. 7( b) illustrates two electrode grids that may be formed using theprocess of FIGS. 5 and 6.

FIG. 8 shows a metal grid on Silicon substrate fabricated usingnanosphere lithography.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a photovoltaic (PV) device, or solar cell, 10.The PV device 10 includes a back electrode layer 12, a PV material 14and a front electrode 16. Light energy is transmitted to the PV layer14, where it is absorbed and transformed into electric energy. Theelectricity generated within the PV device 10 migrates to either thefront electrode 16 or the back electrode 12, from where it is directedout of the cell through an electrical contact 20 or 22. The PV layer 14may be constructed of any among many different types of materials,including, but not limited to, semiconductor junctions, organic-dyebased materials, photoelectrochemical cells, polymer solar cells,nanocrystal solar cells or dye sensitized solar cells, as well as otherPV cell technologies.

More specifically, in the device of FIG. 1, the PV material 14 may be asemiconductor substrate comprised of a polycrystalline silicon but canalso be a single-crystalline silicon, and it is of a p-type with, forexample, an impurity concentration of 5×10¹⁶ cm⁻³ with boron as theimpurity, for example. The thickness of this semiconductor substrate 14may be from 1 μm to 150 μm. An aluminum oxide (Al₂O₃) diffuse-reflectionsubstrate (not shown) that is 800 μm thick, for example, may be adheredto the back surface of the semiconductor substrate 14.

As mentioned above, series resistance associated with the contactelectrode is a serious limiting factor for high performance solar cells.FIG. 2( a) illustrates the impact of the varying electrode thickness (orthe electrode series resistance) to the cell efficiency. Thusengineering an optimum contact electrode is critical in achieving peakperformance for all type solar cells.

The overall cell series resistance originates mainly from threecomponents as shown in FIG. 2( b): emitter layer resistance (R_(E)),electrode (busbar and finger) resistance R_(BF) and theelectrode-semiconductor contact resistance (R_(C)). Other than thesefactors, the shadowing loss due to the electrode coverage (typically˜10%) also accounts for the loss of the cell performance. These seriesresistances could be reduced by increasing the area coverage but thiswill increase the shadowing loss. One could also make the electrodethicker, but this is often limited by the deposition technique.

Ideally a solar cell needs a contact electrode that is transparent buthighly conductive to minimize the shadowing loss and series resistancelosses. One attempt is to use transparent conducting oxide film such asindium tin oxide (typical transmission ˜80% and sheet resistance ˜20Ω/sq). The present invention provides an alternative technique,utilizing an electrode grid that reduces both series resistance andeffectively reduces the shadowing loss of the electrode.

With reference to FIG. 3, in accordance with an embodiment of thisinvention, the finger electrode 30 is replaced with a “holey” metal grid32 with small feature size while keeping the same shadowing factor. Thegrid 32 can have holes 34 with both subwavelength dimensions (w<λ), aswell as superwavelength dimensions (w>λ). The hole pattern may beperiodic, may be aperiodic, or may be partially periodic. In the case ofperiodicity, the lattice period α may be, for example, ˜200 nm to ˜50μm.

Such an electrode structure 32 provides important advantages. Onesignificant advantage is reduced emitter series resistance (R_(E)),since the current path from the junction to the electrode are reduced.E.g. typical finger spacing of solar cell is 500 μm and the emitterseries resistance scales with this value. By having a metal grid 32 withvery small lattice spacing (e.g. in order of μm) the series resistancecan be reduced significantly. Another important advantage is that, inthe case where the hole dimensions are larger than the wavelength oflight (w>λ), the metal will cover <20% of the area, resulting in >80%transmission. At the same time, the conductivity of the film will behigh. As a result, these conducting holey films can be used both forthin film PV (as a replacement for ITO or doped ZnO) and for crystallinesilicon. In the case of crystalline Si, since the lateral path for theminority carrier in the silicon is shorter, it is possible to dope theemitter to lower values, thereby improving minority carrier lifetimes inthe emitter.

In the case where the hole dimension is smaller than the lightwavelength (w<λ), since the grid has linewidth smaller than thewavelength of light, the grid lines will be less visible to the incominglight or the reflection will be reduced. This effectively reduces theshadowing loss due to the reflection of the electrode. Further, when thegrid lattice period is smaller than the light's wavelength (α<λ),embodiments of the invention can exploit a plasmonics effect of enhancedtransmission phenomena through an array of subwavelength holes. Thiseffect occurs because the periodicity of the metal structure helps tocouple the light and the surface plasmons more effectively and reradiatethe light energy as transmitted light. This feature is illustrated inFIGS. 4( a) and 4(b). This effect could also potentially reduce theshadowing loss due to the electrode.

Fabrication of a structure with submicron structures over a large areais prohibitively expensive using standard lithography technique (deep UVor electron beam lithography). Thus a nanosphere lithography techniquemay be used to realize large scale but low cost patterning of asubwavelength metal grid for solar cell application.

FIGS. 5 and 6 illustrate, as an example, a fabrication process that maybe used to form the holey electrode grid. In this process, at step 42,the nanospheres (NS) colloid is prepared with proper dilution withmethanol (added with surfactant Triton-X 100 by 1:400 volume) to achievemonolayer deposition. The amount is, in one embodiment, determinedexperimentally (e.g. for 500 nm NSs NS:methanol=7:5 volume). At step 46,the nanospheres 48 are spun on the substrate 50, as shown at 52. Thesize of the NS are trimmed at step 54 to the desired size using O₂plasma. The etch timing data is shown in FIG. 7( a). At step 56, metal60 is deposited onto the substrate. At step 62, the NSs are lifted offthe substrate. This may be done by first peeling the NSs off using tapeand then cleaning using NMP (N-Methyl-2-pyrrolidone).

A result of a metal grid 64 made by nanosphere lithography is shown inFIG. 8. Note that the holes filling factor can be controlled bycontrolling the size of the NSs with O₂ plasma trimming process as shownat 66 in FIG. 7( b). A larger filling factor means smaller shadowingloss. For a closely packed NS lattice as shown in FIG. 7( b), theminimum shadowing factor (area covered by metal) is 1−√{square root over(3)}π/2 or 9.3%. However since the feature size of the metal lines aresmaller than the wavelength of light, the effective shadowing loss willbe smaller.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects stated above, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art, and it is intended that the appended claims coverall such modifications and embodiments as fall within the true spiritand scope.

1. A method of forming an electrode grid on a photovoltaic semiconductorsubstrate, comprising: providing a photovoltare semiconductor substratehaving first and second opposite sides; forming a monolayer ofnanospheres, in a nanospheres colloid, on a surface of only one of thefirst and second sides of said substrate; trimming the nanospheres to adesired size depositing a layer of a metal on the substrate and thenanospheres to form a metallic layer on the substrate, around saidnanospheres; and removing said nanospheres from the metallic layer toform the metallic layer into an electrode grid defining a multitude ofholes on the substrate and electrically connected thereto; wherein saidmultitude of holes have circular shapes with diameters between 100 and1000 nms, said holes form a periodic pattern in said electrode grid, andsaid pattern repeats with a period between 200 nms and 50 μms; andwherein the shape, size and pattern of the holes of the electrode gridprovide the electrode grid with greater than 80% transmission of lightthrough the electrode grid.
 2. The method according to claim 1, whereinsaid trimming includes trimming said nanospheres using O₂.
 3. The methodaccording to claim 1, wherein said removing includes lifting thenanospheres off said substrate.
 4. The method according to claim 3,wherein said lifting includes peeling the nanospheres off said substrateusing a tape.
 5. The method according to claim 1, wherein the electrodegrid, due to the shape, size and pattern of the holes of the electrodegrid, couples light, using a surface plasmon effect, to reradiate lightenergy as transmitted light.